1. Field of the Invention
The present invention relates to a focus detection device for an autofocus which is used in a single-lens reflex camera or the like.
2. Description of the Related Art
As a focus detection device for an autofocus, there is a passive system in which luminance information of an object is received by a photosensor and a focal position is detected by an electric process. A phase difference detection system in the passive system is a system in which a lateral deviation of luminance signals in two sets of photosensitive elements is detected.
In the autofocus (hereinbelow, referred to as an AF) of the phase difference detection system which is used in a single-lens reflex camera, there is such a problem that a focus detecting position differs depending on a color temperature of an object light source. Such a problem occurs by a color aberration of a lens as a cause. An example of a focus detection device which can solve such a problem has been disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. S63-168613).
The device has an AF sensor for detecting a focal point and a color temperature sensor and has a correcting unit for correcting an output of the AF sensor based on an output of the color temperature sensor. A photosensor array of the AF sensor and a pair of photodiodes of the color temperature sensor are adjacently arranged in an AF sensor module.
There is an example as disclosed in Patent Document 2 (Japanese Patent Application Laid-Open No. 2005-300756) in which automatic gain control is provided for the focus detection device having the AF sensor and the color temperature sensor in order to improve characteristics of each sensor.
It is demanded to realize a high AF speed owing to the improvement of a camera frame speed or the like. As a method of realizing the high AF speed, there is a method whereby a difference between the maximum value and the minimum value of the sensor output is detected and accumulation control of the AF sensor is made (hereinbelow, such a method is referred to as contrast control). By using such contrast control, even when a luminance of the object is low, if a contrast is sufficient, the accumulation is finished in a short time and the high speed AF can be performed.
As an example of the contrast control AF, there is Patent Document 3 (Japanese Patent Application Laid-Open No. 2000-180706).
Therefore, in the present AF, it is demanded to realize a high precision using the color temperature sensor and the like and a high speed using the contrast control or the like.
However, in Patent Documents 1 and 2, nothing is disclosed about the realization of the high speed by the contrast control. In Patent Document 3, although the contrast control is used for the accumulation control of the AF sensor, there is a possibility that the AF speed cannot be improved depending on the object.
This is because of the following reasons. In the object of a low luminance and a high contrast, the accumulation of the AF sensor is finished relatively in a short time due to the contrast control. As for the color temperature sensor, the maximum value of the sensor output is detected and accumulation control is made (hereinbelow, such control is referred to as peak control). Therefore, in the case of the object of the low luminance, an accumulation time period becomes long. Since outputs of both of the AF sensor and the color temperature sensor are necessary for the AF operation, the AF speed is eventually limited by the accumulation time period of the color temperature sensor.